1. Field of the Invention
This invention relates to aqueous, alkaline liquid solutions consisting essentially of water, at least about 75 grams/liter of NaMnO.sub.4, and NaOH in an amount sufficient so that the removal of substantially all of the manganese residues from the surface of a material contacted with said solution can be accomplished by acid neutralization only. This invention also relates to a process of removing resin smears on interior walls of holes formed in insulating substances and/or etching the interior walls of such holes using the aqueous, alkaline sodium permanganate solutions of this invention.
2. Description of the Prior Art
Hole forming operations in resin containing materials often result in the smearing of resin over the interior wall or barrel of a hole. This resin smear is primarily attributable to the generation or utilization of temperatures exceeding the melting point of a resinous component of the material during the hole forming process.
Where holes are drilled in epoxy impregnated fiber glass laminate materials (such as those employed to make printed circuit boards), friction of the drill bit against the material raises the temperature of the bit. Often, drill bit temperatures are generated which exceed the melting temperature of many resin systems. The drill bit thus picks up melted resin on its course through the material being drilled, and this melted accretion is smeared in the barrel of the hole. In laser drilling to contact interior conductors in organic insulating substrates, a similar resin accretion or smear can develop on the exposed conductor surface.
While the problem of resin smear on the hole walls may be ignored in some applications, it is at times imperative that it be removed. For example, when a multilayer circuit board is made, holes are formed in a resin-containing material which includes a plurality of parallel planar metallic conductors, with the hole perpendicular to, and communicating with, two or more parallel metallic conductors. It is often desired to metallize the hole walls in order to form a conductive path between two or more of the metallic conductors, so the resin smear must be removed from the edges of the hole through the metallic conductors if conductive contact between the metallized hole wall and the metallic conductors is to be achieved. Thus, when circuit board holes are drilled through a copper clad base plastic laminate or through a plastic laminate containing internal conductor planes such as in a multilayer circuit board, resin smear on the metallic surfaces exposed to the walls of the holes must be removed to achieve proper functioning of the plated through-holes.
Plated through-holes as described above are useful as electrical connections between printed circuits having metallic conductors on both sides of the plastic laminate or between two or more of the various planes and surface conductor layers in multilayer boards. The electrical and mechanical integrity required for this function can only be attained by insuring complete removal of resinous materials from the entire inner circumference of the portion of the metallic conductor exposed by the hole.
Numerous methods are known for removing resin smear. One approach is a mechanical one and involves channeling a dry or wet stream of abrasive particles through such holes. A similar method is the use of hydraulic pressure to force a thick slurry of abrasive material through the holes. However, these mechanical methods are generally slow and difficult to control. Furthermore, complete removal or smear in all holes in a given circuit board is difficult to achieve.
Chemical methods have also been used to desmear holes. Generally, the chemicals used attack the smeared resinous coating. The chemicals include, for example, concentrated sulfuric acid (down to about 90 percent concentration). Smeared epoxy resin, which is usually less than about 0.001 inch thick, can be removed with about one minute's treatment with concentrated sulfuric acid. Unfortunately, the high sulfuric acid concentration required is very hazardous and requires extraordinary precautions by operators. Also, undesirably rough holes are produced. In addition, the concentrated sulfuric acid rapidly absorbs water, which limits its useful life span and can cause variations in the immersion times required to desmear the holes.
Another chemical smear removal agent which has been employed is chromic acid. While it is easier to control etch rates with chromic acid because it is a slower etchant that concentrated sulfuric acid, it still requires operator caution and special equipment. Chromic acid also presents toxicity and waste disposal problems.
Permanganate has also been used for smear removal and for etching of various materials. For example, U.S. Pat. No. 1,969,678, issued Aug. 7, 1934 to White et al. discloses etching solutions for metals, e.g. copper, which contain ferric chloride and an oxidizer which converts ferrous ions to ferric ions. This oxidizer may be KMnO.sub.4. Of course, these etching solutions would have at best limited utility in preparing circuit boards since the boards often contain a copper layer which would be subject to removal by the ferric chloride.
U.S. Pat. No. 3,293,148, issued Dec. 20, 1966 to Dell et al., discloses a cleaning solution for metals which contains an oxidizer, alkali metal hydroxide, gluconic acid and polyethylene glycol. Among the many oxidizers disclosed are permanganates, although they are not preferred and no example is given employing them.
U.S. Pat. No. 3,425,947, issued Feb. 4, 1969 to Rausch et al., relates to solid concentrates useful in preparing treatment baths for removing organic substances from metal surfaces. The concentrates contain about 50-90% alkali metal hydroxide, 10-50% permanganate and 1 gram equivalent/gram mole of permanganate of a reducing agent. Typical solutions prepared by the solid concentrates contain about 105 g/l of KMnO.sub.4 (which is completely soluble only at elevated temperature)and about 250 g/l NaOH.
U.S. Pat. No. 3,457,107, issued July 22, 1969 to Mickelson et al., discloses compositions for chemically polishing stainless steel. These compositions are aqueous solutions containing alkali metal hydroxides, e.g. NaOH, and alkali metal permanganate, e.g. KMnO.sub.4. Typical solutions contain about 39 g/l NaOH, 16 g/l KMnO.sub.4 and 4.8 g/l Na.sub.2 CO.sub.3 ; or 78 g/l NaOH, 32 g/l KMnO.sub.4 and 9.6 g/l Na.sub.2 CO.sub.3.
U.S. Pat. No. 3,489,625, issued Jan. 13, 1970 to Dell et al., discloses acid pickling of carbon steel followed by treatment with an alkaline permanganate solution. The alkaline permanganate solution contains about 5-50 g/l KMnO.sub.4 and about 10-200 g/l alkali metal hydroxide.
U.S. Pat. No. 3,506,397, issued Apr. 14, 1970 to Vincent et al., relates to the treatment of ABS resin with a composition containing phosphoric acid and potassium permanganate.
U.S. Pat. No. 3,598,630 and 3,647,699, issued respectively on Aug. 10, 1971 and Mar. 7, 1972 to Doty et al., are also related to phosphoric acid/potassium permanganate solutions for treating ABS polymers.
U.S. Pat. No. 3,625,758, issued Dec. 7, 1971 to Stahl et al., deals with the manufacture of printed circuit boards. As one step in the manufacturing process, the board is treated with an oxidization and degradating agent which may be a permanganate solution. However, no details or specific examples regarding the composition of the permanganate solution are provided.
U.S. Pat. No. 3,652,351, issued Mar. 28, 1972 to Guisti, discloses compositions for etching synthetic polymers. These etchant compositions contain 2.5-8.5% potassium and/or sodium manganate, 30-70% sodium and/or potassium hydroxide and 21.5-67.5% water. These compositions are prepared from the corresponding permanganate which is reacted with a large amount of sodium and/or potassium hydroxide at elevated temperature to reduce the permanganate to manganate. For instance, in Example 1 Guisti combines 50 g KOH, 5 g KMnO.sub.4 and 20 g water and heats the resulting mixture at 80.degree.-90.degree. C. until the deoxidation reaction is complete. Likewise, in Example 2 the reaction mixture contains 45 g NaOH, 6 g NaMnO.sub.4 and 20 g water, and Example 3 employs 25 g NaOH, 25 g KMnO.sub.4, 2.5 g NaMnO.sub.4 and 20 g water.
Attempts were made to duplicate Guisti's examples and it was found that the permanganate-containing reaction mixtures were not liquid solutions but rather paste-like materials which crystallized even at boiling temperatures. This is perhaps not surprising, however, since the reaction mixtures contain vast quantities of alkali metal hydroxide. More particularly, the reaction mixture for Example 1 contains (on a grams/liter basis) 1000 g/l KOH and 105 g/l KMnO.sub.4 ; Example 2 uses 990 g/l NaOH and 132 g/l NaMnO.sub.4 ; and Example 3 uses 575 g/l NaOH, 575 g/l KOH, 108 g/l NaMnO.sub.4 and 108 g/l KMnO.sub.4. Finally, it was found that the manganate compositions produced by duplicating Guisti's examples were not satisfactory etchants.
U.S. Pat. No. 3,833,414, issued Sept. 3, 1974 to Grisik et al., discloses a method for removal of aluminide coatings from metal by treating the metal with a mixed acid solution, followed by treatment with an alkaline permanganate aqueous solution, and finally a second treatment with the mixed acid. The alkaline permanganate aqueous solution contains 8-11% NaOH, 8-11% Na.sub.2 CO.sub.3 and 4-6% KMnO.sub.4.
U.S. Pat. Nos. 4,042,729; 4,054,693 and 4,073,740, issued respectively on Aug. 16, 1977; Oct. 18, 1977 and Feb. 14, 1978 to Polichette et al., relate to treating resinous surfaces with a solution of manganate (MnO.sub.4.sup.2-) and permangante (MnO.sub.4 -) where the molar ratio of manganate/permanganate is up to 1.2 and the pH of the solution is 11-13. Polichette et al. teach that higher pH increases the MnO.sub.4.sup.2- /Mn.sub.4.sup.- ratio, which is undesirable.
U.S. Pat. No. 4,294,651, issued Oct. 13, 1981 to Ohmura, discloses etching of a semiconductor substrate with a composition containing a fluorine compound (7-38%), an oxidizing agent such as KMnO.sub.4 (2.5-7%) and alkali such as KOH or NaOH (1-10%).
U.S. Pat. No. 4,425,380, issued Jan. 10, 1984 to Duffy et al., discloses a process for removing resin smear (desmearing) from a interior wall of a hole in a resinous substrate. The process involves contacting the substrate with an alkaline permanganate solution, having a pH between 11 and 13, at elevated temperature. With regard to the permanganate, it is stated that any metal salt of permanganic acid which is stable and soluble to the extent of at least 10 g/l in water can be employed, sodium permanganate and potassium permanganate being preferred. While it is disclosed that amounts from about 10 g/l up to the limit of solubility of permanganate may be employed, it is emphasized that especially good results are obtained with NaMnO.sub.4 or KMnO.sub.4 in the range of 10-60 g/l. Further, it is taught that the rate of bonding sites formation increases up to about 60 g/l, but no further increases in rate is noted above this level. Thus, Duffy et al. place a practical upper limit of 60 g/l of permanganate on their desmearing solutions.
The desmearing solutions disclosed by Duffy et al. also have a narrow pH range of 11-13. Duffy et al. state repeatedly that a higher pH leads to an assortment of severe problems. For example, they state at col. 3, lines 2-5 that the use of high pH permanganate solution results in localized inactive areas when used to desmear epoxy resin and etch back polymide in circuit boards. These inactive areas lead to the formation of pin holes or plating voids when the board is subsequently electrolessly plated with metal. This problem of pin holes and plating voids after cleaning with high pH permanganate solutions is referred to again at col. 3 lines 37-40.
Duffy et al. point to other problems caused by high pH, i.e. above pH13, permanganate solutions. At col. 4, lines 27-29 they state that large amounts of residual manganese are found when alkaline permanganate treating solutions having a pH above 13 are used. They further state at col. 4, lines 31-38 that when these high pH permanganate solutions deposited on a substrate were not neutralized, subsequent electroless metal deposition was rapid but the electroless metal bath spontaneously decomposed. On the other hand, when the high pH permanganate treating solution was neutralized, residues remained on the substrate resulting in voids in the hole when a metal was subsequently electrolessly deposited thereon. Thus, the clear inference from the teachings of Duffy, et al. is that high pH permanganate solutions would be expected to perform poorly, if at all, in a desmearing process, largely due to their inability to provide a substrate which can be electrolessly plated satisfactorily.
The problem of manganese residues deposited on the substrates when high pH permanganate solutions are employed is specifically addressed by Duffy, et al. in their Examples. There, it is demonstrated that when a high pH KMnO.sub.4 solution is used relatively large amounts of manganese residue are left on the substrate. According to Duffy et al. these manganese residues can not be removed simply with an acid neutralization step. An additional step involving immersion in an alkaline solution is required to remove substantially all of the manganese residues.
U.S. Pat. No. 4,430,154, issued Feb. 7, 1984 to Stahl et al., relates to a method for removing an adhesive medium from printed circuit boards without corroding the base material or copper conductor on the board by treatment with an alkaline permanganate or chromic acid solution. The alkaline permanganate solution disclosed contains 50 g/l KMnO.sub.4 and 50 g/l NaOH.
British Pat. No. 1,479,558 of Kollmorgen Corporation, published on July 13, 1977, also relates to desmearing and etchback of printed circuit boards and wire conductors by treatment with an alkaline permanganate solution containing potassium permanganate, a strong base (e.g. NaOH) and a fluorinated hydrocarbon wetting agent. The solution contains about 10-75 g/l KMnO.sub.4 and enough NaOH to achieve a pH of 13-14. Typically, about 40 g/l of NaOH is employed. The desmearing process is conducted at temperatures from about 35.degree.-50.degree. C., it being disclosed that temperatures above 70.degree. C. result in increased permanganate demand in order to maintain the bath composition without yielding any apparent advantage, i.e. the overall process becomes less efficient in terms of permanganate consumption.
Thus, it can be seen from the foregoing that aqueous permanganate solutions are well known for a variety of uses. However, the art has focused almost exclusively upon solutions containing potassium permanganate. Unfortunately, however, KMnO.sub.4 has very limited solubility in water, being soluble in water at 20.degree. C. at a maximum of only about 63.8 g/l. Also, even at its saturation point KMnO.sub.4 is a relatively slow etchant.
In an effort to improve the etching efficiency of KMnO.sub.4 the art has in recent years turned to highly alkaline KMnO.sub.4 solutions. In particular, U.S. Pat. No. 4,425,380 and British Pat. No. 1,479,553 discussed above deal with KMnO.sub.4 solutions having a pH of 11-13 and 13-14 respectively. While this high alkalinity did appear to confer some benefits upon the KMnO.sub.4 solutions, it leads to other problems such as the deposition of manganese residues which are difficult to remove. For this reason, it was believed that KMnO.sub.4 solutions having a pH above about 13 would not produce satisfactory etching results (see the above discussion of U.S. Pat. No. 4,425,380).
Increased etching efficiency has also become desirable because in many situations it is necessary that the etchant composition not only be capable of desmearing small areas of resin, but also be capable of "etching back" the resin. The term "etchback" is used in the art to refer to the removal of resin from, for example, the barrel of a hole in a printed circuit board to expose a portion of the layers of copper or the like. In essence then, the copper layer protrudes slightly into the hole in the board. While for practical purposes etchback is any resin removal which is greater than simple desmearing, in some cases rather specific etchback requirements are necessary. (For instance, the military defines etchback, for their purposes, as the removal of 0.3-3.0 mils of resin.) While there is no definitive line between desmearing and etchback, for the purposes of this application, a solution which causes a weight loss in excess of 25 mg. (or approximately 0.05 mil) after 15 minutes of etching a 3 in.times.3 in epoxy/glass circuit board are considered to cause etchback. Etchant compositions which are capable of removing this much material in a reasonable time without being uncontrollably agressive has been difficult to find.
It has now been discovered that many of the problems encountered with the prior art KMnO.sub.4 solutions can be overcome completely with aqueous alkaline solutions of NaMnO.sub.4 and NaOH. The NaMnO.sub.4 is vastly more soluble in water than is KMnO.sub.4, easily producing aqueous solutions containing 400-500 g/l NaMnO.sub.4 at 20.degree. C. Also, while the art teaches away from high pH solutions, it has now been found that highly alkaline solutions of NaMnO.sub.4 and NaOH actually solve many of the problems which the prior art teaches with high pH KMnO.sub.4 solutions. Desmearing and etch rates found with alkaline NaMnO.sub.4 /NaOH solutions are much faster than with KMnO.sub.4 solutions, and while manganese deposits still occur with NaMnO.sub.4 /NaOH solutions, the deposits are readily removed with a simple acid neutralization, as opposed to the far more complicated treatment required with KMnO.sub.4 solutions. Furthermore, these NaMnO.sub.4 /NaOH solutions provide excellent, readily controlled etchback.